1. Field of the Invention
The present invention relates to a semiconductor device for emitting, upon receiving a light energy, a highly spin-polarized electron beam.
2. Related Art Statement
Spin-polarized electron beam in which a large or major portion of the electrons have their spins aligned in one of the two spin directions, is used in the field of high-energy elementary-particle experiment, for investigating the magnetic structure of atomic nucleus or the magnetic structure of material's surface. For generating a spin-polarized electron beam, it is commonly practiced to apply a circularly polarized laser beam to the surface of a compound semiconductor crystal such as of gallium arsenide GaAs, so that the semiconductor crystal emits an electron beam in which the spin directions of the electrons are largely aligned in one of the two directions because of the selective transition due to the law of conservation of angular momentum.
However, it is theoretically estimated that the above-indicated conventional, spin-polarized electron beam emitting device would suffer from an upper limit, 50%, to polarization (degree of polarity) of the spin-polarized electron beam emitted therefrom, at which limit the ratio of the number of electrons having upspins to the number of electrons having downspins is 1 to 3, or 3 to 1. In addition, it is technically difficult to achieve the theoretical upper limit of 50% because of various sorts of restrictions, and accordingly only a polarization of about 40% at most is available. Thus, the conventional semiconductor device is not capable of producing a highly spin-polarized electron beam having a not less than 50% polarization.
Meanwhile, it is possible to provide a spin-polarized electron beam emitting device in which a semiconductor crystal has a stress in a certain direction so as to have a uniaxial anisotropy in the valence band thereof. However, it is difficult to cause the semiconductor crystal to have a sufficiently large strain or cause the crystal to have a strain in a stable manner. In addition, this device would suffer from the problem that an external means used for producing the stress or strain in the semiconductor crystal may interfere with extraction of the spin-polarized electron beam therefrom.